Titanium alloy has a light weight, high strength, and high corrosion resistance, so has been taken note of as a material best suited to extreme environments of large depths, high temperatures, high pressures, and high corrosion in geothermal development, deep ocean floor oil field and gas field development, etc. In particular, α+β titanium alloy, which is frequently used for aircraft applications and boasts a good record, and high corrosion resistance α+β titanium alloy, in which a small amount of Pd or Ru of about 0.01 to 0.3% is added to improve the corrosion resistance, are considered promising as particularly superior materials for extreme environments. In such applications, pipes are the main product form.
In the past, as the methods for production of α+β titanium alloy pipes, there have been the following A to D:                A: The method of hot extrusion (seamless pipe),        B: The method of using a plug mill etc. for continuously piercing, elongating, sizing, drawing, and other rolling to form pipes (seamless pipe),        C: The method of cold forming thick plate by the method of rolling known as roll bending and welding together the abutted plate ends (welded pipe), and        D: The method of hot or warm press forming thick plate to a semicircular sectional shape and welding together two of these (welded pipe).        
Among these, the method of C is not used much at all for the production of titanium alloy pipe since circularity of the pipe is difficult to secure, long pipes are difficult to obtain, etc. Further, the method of D requires heating the material for the hot or warm forming. Further, welding is required at two locations, so the overall cost becomes high. This method is therefore also seldom used.
On the other hand, the methods of A and B have the advantages of enabling production of seamless pipe without weld zones where deterioration of the characteristics is feared. Use is mainly made of titanium alloy pipes produced by these methods.
Titanium alloy pipes made by these methods, however, often have deep flaws or defects at the inside and outside surfaces. A considerable amount of machining is required to remove these flaws or defects. Titanium alloy, however, has an extremely poor machinability. There was therefore the problem that this large amount of machining led to a major increase in cost.
Further, pipes produced by these methods had the problems of a large unevenness of wall thickness and therefore was easily formed with portions of extreme thickness and portions of thinness of the pipe. That is, since it was necessary to design the strength for the wall thickness of the thinnest portion, the material of the portions thicker than that was wastefully used. Not only did the cost become high, but also there was the problem that the characteristics of titanium alloy, i.e., the light weight, could not be sufficiently manifested. Of course, it is possible to even out the wall thickness by machining, but as explained above, titanium alloy is a material with an extremely poor machinability and the cost ends up increasing tremendously. Therefore, it was de facto difficult to eliminate the large unevenness of wall thickness.